Dust devils and wind scours on ripples on dunes

A piece of Mars: So much wind. There are dark swirly tracks of dust devils that have passed by, ripples covering dunes, wind scours around rocks, and of course dunes. Dune crests have a different color than other regions: are they less covered in dust? made of a more grayish sand that is more easily blown up the dune by the wind? or both? (HiRISE ESP_040885_1295, NASA/JPL/Univ. of Arizona)